Revert "Make UserNotLoggedIn redirect to login page"

Lots of small issues that could be very annoying:

* Usability regression: the user is not informed *why* they were
  redirected to the login screen. Not even a generic "You must log in
  to complete this action".
* When testing this on my watchlist, after logging in I am now redirected
  to http://localhost/wiki/index.php?title=Watchlist&title=Special%3AWatchlist
  (the title is repeated twice and one of them is incorrect). This
  worked correctly before.
* SpecialPage::requireLogin() needs documentation and code updates.
* A whole ton of messages used in these prompts is now unused.
* Probably needs release notes.

I don't have the time to fix up myself right now, and a WMF
branch cut is looming. Let's just try this again later.

This reverts commit e0af129cbdb7421b4d9d7858cc63125e12f35a4a.

Bug: 15484
Change-Id: I728e477cf6e10b90846ee0b2d9e5087bc3a0af3e
Merge "Revert "Make UserNotLoggedIn redirect to login page""